Don't return Quantities from high_level_objects_to_values

[astrofrog]

The high_level_objects_to_values function should not be returning quantities because otherwise this is resulting in quantities being passed through to world_to_pixel_values which should not happen (only bare scalars/arrays should be used).

I have also added a conversion of units to world_axis_units but I'm not sure if we should. This is a possible pragmatic solution to the fact that some WCSes might define world_axis_object_components in a way that is inconsistent with world_axis_units, such as in spacetelescope/gwcs#495 - however, if all the metadata was defined correctly this should not be needed. So another option is to be strict here and simply replace .to_value() by .value.

I still need to add a test for this, will do once we agree on the approach.

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[astrofrog]

@Cadair @nden I've updated this to actually just check the input/output types of the high level <-> values functions rather than silently dropping quantity-ness if present. I think it makes sense to be somewhat strict here otherwise introducing quantities into this can cause confusion and errors. This would have caught invalid WCSes like spacetelescope/gwcs#495 at least when used via SlicedLowLevelWCS.

[pllim]

Will this have performance impact? 🤔

[astrofrog]

I very much doubt it, it's only looping over a couple (or maybe up to 5) dimensions - not over each individual value of a large array

[pllim]

FWIW benchmark isn't affected but I also don't know if you have benchmarks for WCS APIs.

BENCHMARKS NOT SIGNIFICANTLY CHANGED.